Finding the substrates of protein kinases that control a wide spectrum of biological functions is still one of the biggest challenges in signal transduction studies today. In this proposal, I use several novel techniques to identify physiological substrates for BRI1. BRI1 is a plasma membrane localized receptor Ser/Thr kinase, playing a crucial role in perceiving plant steroid hormones. Plant steroid hormones, called brassinosteroids (BRs), play an essential role throughout plant development. A large number of BR biosynthetic or metabolic mutants that have been defined in Arabidopsis and other plants have aided in elucidating the brassinolide biosynthetic pathways. In contrast, much less is known about BR signaling components. To date, more than 25 recessive mutant alleles in the BRII locus have been isolated, but no other signaling components have emerged from the loss-of-function mutant screens. On the other hand, several downstream components including nuclear localization proteins and transcriptional factors in the BR signaling have been identified and characterized in Dr. Chory's laboratory. Thus, the BR signaling pathway from the cell surface to the nucleus is beginning to be elucidated. Still, there is a large gap in knowledge of the early components of the BR signaling pathway, particularly direct substrates of BRII 's kinase. I propose to use a combination of chemical genetic, biochemical, molecular and genetic techniques to identify and characterize substrates of BRI1.